Britain's top health official says the government plans to ban blood donations from anyone who has received a blood transfusion since 1980, in an attempt to prevent transmission of the human form of mad cow disease.

Health Secretary John Reid told the British parliament on Tuesday that he is acting on expert advice in banning donations from anyone who received a blood transfusion after January 1, 1980.

Mr. Reid says the action is being taken because of what he calls "an uncertain but slight risk" of contracting the illness from a transfusion. He says the ban will go into effect April 5.

In December, the British health secretary announced the death of a recipient of a blood transfusion from an individual who was found much later to have the disease. The donor died more than three years after donating the blood, and the recipient died more than six years after receiving the transfusion. Mr. Reid said Tuesday it is still not clear whether the second patient contracted the disease from the first.

Experts say mad cow disease, or bovine spongiform encephalopathy, is spread by cattle that eat food containing certain products from other cattle infected with the illness. People who eat contaminated meat can come down with the human variant, Creutzfeldt-Jakob disease, which leads to paralysis and death.

More than 120 people in Britain are known to have died from the human form of mad cow disease since 1996, with individual cases reported in several other countries.